Semiconductor fabrication process uses many types of gases including dilute gases and special material gases. As the scale of integration in a semiconductor is increased, higher purification is required for such gases. For special material gases, reaction with water content at room temperature generates reaction products, which may cause contamination and corrosion in the gas supply system or process chambers, resulting in particle generation occasionally. Since the minute patterning intervals in a semiconductor circuit are required to have an accuracy of the order of sub microns, such particle generation is undesirable for the fabrication.
With the required purity of the gas used in the semiconductor fabrication process thus becoming increasingly higher, strict requirements are set for the quality of the members for gas piping which supplies high purity gas to the use point in the semiconductor fabrication. Pipe members are required to minimize desorption of water content, metal elements and fine particles.
Conventional method to meet such requirements is to use stainless steel with its internal surface contacting with gases finished with bright-annealing as the piping member or finished with electrolytic polishing which is used recently. Electrolytic finishing improves the smoothness of the surface in contact with the gas and reduces adsorption and desorption of gases resulting in particles such as dusts and corrosive products decreased. The electrolytic polished parts tend to be used more and more.
However, even provided with electrolytic polishing, stainless steel comprising Fe, Cr and Ni is always susceptible to elution of such metal ions. Japanese Patent Application Laid-open No. 31956/1989 discloses measures to suppress elution of metal ions, where the corrosion resistance is improved by thermal treatment of electrolytically polished surface at a temperature from 280.degree. to 580.degree. C. in the atmosphere of 25% or more of oxygen content to form an oxidized coating.
Though the corrosion resistance is certainly improved by the measures disclosed above, highly corrosive gases may corrode the oxidized film coating on the surface, causing the constituents such as Fe, Cr and Ni to elute. The corrosion resistance, and particularly the pitting resistance are still insufficient in such cases.
Further, the stainless steel for semiconductor fabrication equipment is particularly required to have superior water desorption property in addition to corrosion resistance. Specifically, gases for semiconductor fabrication equipment and water desorbed from piping as a gas constituent may cause hydrolysis, generating hydrochloric acid and hydrofluoric acid, which can corrode metal members. Therefore, it is required that the stainless steel for semiconductor fabrication equipment desorbs only a little water. To meet the requirement, Japanese Patent Application Laid-open No. 198463/1989 suggests a method to provide heat oxidization with controlling the dew point of water. However, coating formed by oxidization has much Fe oxide and the stainless steel member with such coating does not have an excellent corrosion resistance.
Thus, there has not been a stainless steel member for semiconductor fabrication equipment with sufficient corrosion resistance and water desorption property at a time.